Generally, polymer materials are widely used in space-aviation industries, automobile industries, construction industries and household appliance industries and the like because they have high strength and hardness, excellent durability and formability in spite of low density and weight. With the advancement of industry, demands for the polymer materials are increasing, and thus conventional industrial materials are being replaced by the polymer materials. Since a polymer is a typical insulator, research into a polymer composite material having excellent electro-conductivity is being made in various directions.
An electro-conductive polymer composite material, which is formed by adding an electro-conductive filler to a polymer resin, is a highly-functional material which can replace metals because it is lighter and more flexible than them. Further, the electro-conductive polymer composite material is frequently used as a thermal interface material (TIM) requiring low electric resistance and dielectric constant because its formation of thermal and electrical conductive networks is feasible and material properties can be easily adjusted according to the shape and amount of a filler.
Electromagnetic radiation comes from natural or artificial sources. The natural sources include lightning, sunspots, auroras and the like, and the artificial sources include all digital electronic equipment including integrated circuits, microprocessors, timers and the like. That is, electromagnetic radiation is caused by an induced current formed on a conductor. Electronic parts malfunction because they confuse this induced current with a normal current, and such a phenomenon is called electromagnetic interference (EMI). EMI shielding reduce the electromagnetic interference (EMI) by reflecting or absorbing electromagnetic waves by using electronic parts made of conductive materials or coating the electronic parts with the conductive materials. A coating process for shielding the electromagnetic interference (EMI) can be used for coating whole equipments, electronic circuits and devices, respectively. An electrostatic dissipation (ESD) coating process is well known to be used to treat electronic parts. For example, the electrostatic dissipation (ESD) coating process is applied to a disk drive head. This disk drive head depends on giant megnetoresistive effect (GMR), and is configured to be easily broken by electrostatic sparks. As the speed of a disk drive increases, electrostatic dissipation (ESD) is more strongly required. In the electrostatic dissipation (ESD) coating process, an electro-conductive additive must be sufficiently dissipated, exact conductivity must be realized, and sloughing must not occur. The electrostatic dissipation (ESD) coating process is frequently used even in the field of packaging. It is quite possible for an ESD market in which carbon nanotubes are used as an electro-conductive additive to be enlarged according to the expansion of the disk drive market. In particular, as computers miniaturize, it is expected that the ESD market will become larger.
A steel sheet which is surface-treated with a polymer coating material is frequently used as a structural member, such as a electronic equipment, a household appliance, a case of an OA device, sash, frame and the like. Hence, Japanese steel manufacturers are developing a coating agent for a steel sheet, including a conductive additive and a heat-dissipative additive. However, when a heat-dissipative additive, such as carbon black, and a conductive additive, such as metal powder, which are currently used mainly, are dispersed in a polymer coating resin, the corrosion resistance of the steel sheet becomes poor. Therefore, it is a very important task to impart corrosion resistance to a steel sheet.
Recently, as electronic appliances are more highly-functionalized and miniaturized, the internal temperature of the electronic appliance is increased by the heat generated therein, so that devices installed therein, such as ICs, CPUs, components and the like, may break down, and the lifespan of the electronic appliance may be decreased. Therefore, in the electronics industry, it is very imminent to discharge heat to the outside of the electronic appliance. In particular, carbon black, carbon nanotubes (CNTs), and the like can be used to increase both the electrical conductivity and thermal conductivity of a polymer, and thus it is very important to systematically research such materials.
Further, the interest in electrostatic dissipation materials is increasing because there is a possibility of a monitoring chip exploding and a danger of the monitoring chip being damaged by electricity. Since a coating agent having an electrostatic dissipation function can solve such problems, it is expected that such a coating agent will play an important role in the automobile market in the future.
Carbon black, which is generally used to prepare a coating agent, is advantageous from the economical point of view, but is disadvantageous in that it is difficult to control the conductivity of a coating agent. Meanwhile, when a coating agent needs to be strongly conductive, carbon-steel fibers are used, and when the coating agent needs to be effectively conductive, metallicized particles are used effectively. However, there is a disadvantage in that the carbon-steel fibers or metallicized particles are expensive.
It is known that when migrating antistatic agents or polymers are used to prepare a coating agent, the coating agent has a surface electric resistance of 1012˜1014 ohms/square and is thus used for insulation, and that when inherently conductive polymers (ICPs) or inherently dissipative polymers (IDPs) are used to prepare a coating agent, the coating agent has a surface electric resistance of 104˜1010 ohms/square and is thus used for dissipation, and that when carbon black-based compounds are used to prepare a coating agent, the coating agent has a surface electric resistance of 102˜104 ohms/square and is thus used for conduction, and that when electromagnetic interference compounds or metals are used to prepare a coating agent, the coating agent has a surface electric resistance of 101˜10-6 ohms/square and is thus used for high conduction.
Referring to documents related to conventional coating agents, Korean Patent Registration No. 10-0764340 discloses a heat-dissipative coating agent which is applied on one or both sides of a metal substrate to form a coating layer and thus can be applied to information storage and display devices, such as compact discs (CDs), laser discs (LDs), digital versatile discs (DVDs), compact disc-read only memory (CD-ROM), compact disc-random access memory (CD-RAM), plasma display panels (PDPs), liquid crystal displays (LCDs) and the like. The heat-dissipative coating agent includes a polyester resin as a binder. When the metal substrate coated with the heat-dissipative coating agent is heated to a temperature of 100° C., the integral emissivity of infrared rays (wavelength: 4.5˜5.4 um) is 0.6 or more. Further, 15˜50% of metal powder, such as nickel (Ni) powder and the like, is added to this coating agent to reduce the electric resistance of the coating agent to a level of 10˜100 Ω/square, thus imparting electromagnetic wave shielding properties to the coating agent. However, in this patent document, the corrosion resistance of a coating agent is not described at all.
Currently, in most Korean patents related to the practical application to CRT coating, antistatic coating, active matrix liquid crystal display component coating or the like using a conductive polymer such as polyethylene dioxythiophene (PEDT), polyaniline or the like, which has electro-conductivity without the aid of additives. In this case, surface electric resistance was mostly measured within the range of 100˜1000 Ω/square.
It is known that conductive coating agents including conductive particles are being studied by paint companies. Examples of the conductive particles may include metal particles such as aluminum particles, zinc particles and the like, carbon black particles, carbon nanotube particles, and the like.
The present inventor developed a method of preparing a nanocomposite material by dissipating an inorganic plate-like compound in a thermosetting resin and then exfoliating the dissipated compound, and produced a nanocomposite coating agent having excellent shielding ability and corrosion resistance using the method (Korean Patent Registration No. 10-0604984). Specifically, this method is a method of preparing an anticorrosive coating agent using nanosize-dispersed plate-like clay (MMT) instead of alumina sol used to prepare a conventional coating agent prepared by adding a cross-linking agent and an anticorrosive agent such as alumina sol to a water-dispersible polymer.
Further, the present inventor developed a nanocomposite coating agent having improved corrosion resistance by adding a monomer or polymer including organic nanosize-dispersed plate-like clay (MMT) and a curing agent to a conventional anticorrosive coating agent prepared by adding other additives to main components including a monomer for an organic solvent, a polymer having reaction groups and a curing agent (Korean Patent Registration No. 10-0872833).
However, a coating agent for a steel sheet, having both conductivity and corrosion resistance, has not yet been disclosed.
Therefore, in consideration of the above problems, the present inventor has attempted to prepare a coating composition for a steel sheet, which can exhibit both conductivity and corrosion resistance. As a result, the present inventor prepared a coating composition having excellent electro-conductivity and corrosion resistance by mixing organo clay, metal powder and carbon black with a base resin and a melamine-based curing agent using ultrasonic waves to form a mixture, uniformly dispersing the mixture and then adding other additives to the dispersed mixture. Thus, the present invention was completed.